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Capacitor check on furnace.

I had looked at furnace with run cap for blower, unit was off on high limit. Did cap test and had 1.2 micro farads(cap rated for 5mfd) Cap wasnt bulged but my question is at 1.2 can I assume cap is at fault not blower? Even though it is not entirely open. Blower was hot. Didnt have cap in van at time so I will have to go back. Tested limit, when things cooled off and it was closed.

Depending on the age/condition of the motor, it may not be a bad idea to replace both the cap and motor. Customers get pissy when you have to go back a few months later to replace the motor. .02

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Someone has been doing his homework, you are correct. I have seen numerous post here in my past years, that say "motors" have a 5%-10% tolerance. I have often wondered if the caps have a tolerance percentage, does this mean that the motor mfg. would say it ok for their motor designed for a 10 mfd. cap, to run at 9 mfd.? Maybe bypass a call to the distributor, straight to the mfg. would get you the correct answer, straight from the horse's mouth.

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Someone has been doing his homework, you are correct. I have seen numerous post here in my past years, that say "motors" have a 5%-10% tolerance. I have often wondered if the caps have a tolerance percentage, does this mean that the motor mfg. would say it ok for their motor designed for a 10 mfd. cap, to run at 9 mfd.? Maybe bypass a call to the distributor, straight to the mfg. would get you the correct answer, straight from the horse's mouth.

I'm unsure about the motors, but can investigate. Most run caps are (+/-) 10% tolerance. So if a motor requires a 10 mfd cap, the cap should read 9 mfd-11 mfd (10%). The motor designers should have this variance factored into the motor design. I would say it would be better to oversize a cap than to undersize it, but not more than 1 size up and only until the proper replacement can be installed.

Reasons:

Efficiency - A capacitor attempts to correct a poor Power Factor by adding Capacitance Reactance. Since a compressor or fan motor is an inductive device with Inductive Reactance, a capacitor will largely zero the reactances, resulting in a compressor or fan motor that runs cooler and uses less electricity. Oversizing or Undersizing screws with the Power Factor on a RUN capacitor.

Rules for Capacitor Replacement:

-Voltage must be EQUAL TO or GREATER than the cap being changed (ie, 370V)
-Start Cap must be (+/-) 20% of mfd
-Run Cap must be (+/-) 10% of mfd
-If the Start Cap is UNDERSIZED, the motor might not start.
-If the Start Cap is OVERSIZED, the high start current might burn out the start winding (Hard Start Kit ???).
-If the Run Cap is incorrectly sized, the Power Factor will suffer, resulting in lower efficiency, higher operating costs, and possible motor damage.

Basically the capacitor values posted on nameplates are engineered to be optimum for peak performance. replace with like part!

Run capacitors are there to correct the single-phase power factor. Three-phase is nearly perfect at 1.00 power factor, but sometimes capacitor banks are used to balance power factor in imbalanced systems.